Currently, fans are usually used for heat dissipation of electronic devices. Oftentimes, a fan includes a blade set and a fan seat. The fan seat has a central tubal portion integrally formed thereon. A pair of bearings made of metal are arranged in the tubal portion. A coil is winded outside of the tubal portion. The blade set is formed with a cap and blades connected to the cap. A magnet pushed by magnetic force of the coil and a stainless steel rotary shaft supported by the pair of bearings are arranged within the cap. As the aforesaid coil generates a magnetic force, the magnet is pushed to cause the blades to operate and produce a wind with a wind pressure. Since the aforesaid stainless steel rotary shaft is arranged on the metal bearings to rotate, after rotating for a period of time, the rotary shaft and bearing will wear and, thus, the lifetime of the fan will be reduced. In order to improve the lifetime of the fan, wear-tolerable ceramic bearings have been developed.
Conventionally, ceramic bearings are installed to a tubal portion of a fan seat as mentioned above. FIGS. 5–6 show a mold 10 for producing a fan seat 20 comprising a tubal portion 22 and a pair of bearings 10 installed in the tubal portion 22. The mold 10 comprises an upper part 12 and a lower part 14. A space 18 having a profile corresponding to the tubal portion 22 and the fan seat 20 is formed between the upper part 12 and the lower part 14 of the mold 10. In producing, molten plastic is injected into the space 18 of the mold 10. After cooled, molten plastic congeals into the tubal portion 22 of the fan seat 20. The bearings 10 are then installed to opposite ends of the tubal portion 22 of the fan seat 20.
However, the tubal portion 22 is prone to deform in cooling process after taken out from the mold 10, which results in the radial size of the tubal portion 22 not consistent along an axial direction of the tubal portion 22. Thus, it is difficult to ensure the pair of bearings 10 precisely coaxial with each other. Friction force between the bearings 10 and the rotary shaft rotating therein is inevitably increased, which results in increasing noise of the fan and reduced lifetime of the fan.